Camera with improved illuminator

ABSTRACT

An illumination and imaging system is provided with a co-axial illuminator that does not include a beamsplitter. The co-axial illuminator achieves efficiencies substantially in excess of those achieved with a beamsplitter. In one aspect, an optical stop is used to reflect illumination upon a target. The optical stop includes an aperture allowing light reflected from the target to pass to a detector. The aperture of the stop has an asymmetrical feature that facilitates operation on specular targets.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to prior provisional applicationNo. 60/255,928 filed Dec. 15, 2000 entitled CO-AXIAL ILLUMINATOR.

COPYRIGHT RESERVATION

[0002] A portion of the disclosure of this patent document containsmaterial which is subject to copyright protection. The copyright ownerhas no objection to the facsimile reproduction by anyone of the patentdocument or the patent disclosure, as it appears in the Patent andTrademark Office patent files or records, but otherwise reserves allcopyright rights whatsoever.

BACKGROUND OF THE INVENTION

[0003] The present invention relates generally to automated opticalassembly and inspection machines.

[0004] These machines are used extensively in the field of surface mounttechnology (SMT). One example of such a machine is known as a pick andplace machine. Pick and place machines are used to automatically placeelectronics components upon a circuit board during manufacture.Generally a component is picked up by a placement head that is movablewith respect to the board. The placement head orients the componentcorrectly and places the component at a desired location on the board.

[0005] In order to properly place components, it is necessary todetermine the initial orientation and location of the component on theplacement head. Additionally, the position of the placement head withrespect to the board itself must be known. Once both are known, theposition and orientation of the component are adjusted to place thecomponent correctly upon the board. In order to sense component positionand alignment, pick and place machines generally employ component align(CA) sensors. In order to sense the position of the placement head withrespect to the board, such machines generally employ board align (BA)cameras. The BA camera is used to image a reference position marker or“fiducial” on the circuit board or board artwork. Analyzing the image ofthe fiducial allows calculation of the BA camera position with respectto the board. Since the BA camera is generally disposed on the placementhead, for “on-head” machines, such analysis provides a relationshipbetween the placement head location and the board.

[0006] As components get smaller, and placement accuracy increases, itis increasingly important to have very accurate component align sensorsand board align cameras. One difficulty in providing accurate images isproviding the correct illumination to the target to be imaged.Illumination can vary based upon the type of image being acquired, thereflectivity of the article, the reflectivity of the background of thearticle, and many other factors.

[0007] One popular method of illumination for board align cameras isknown as co-axial illumination. Co-axial illumination is defined asillumination directed substantially along the line of sight of theimaging optics. Co-axial illumination is useful in that shadow imagesare minimized and surface features normal to the optical axis of thecamera are illuminated efficiently, especially for objects with specularsurfaces. Typically, co-axial illumination is effected using a lightsource projecting illumination from the side of the optical axis into abeamsplitter which bends the beam such that it aligns with the opticalaxis of the camera. This technique suffers from at least twolimitations. First, the beamsplitter is not an economical part and thuscan significantly increase the cost of the system. Second, the outgoingbeam is attenuated by the beamsplitter and substantially decreasesefficiency for diffuse targets.

SUMMARY OF THE INVENTION

[0008] An illumination and imaging system is provided with a co-axialilluminator that does not include a beamsplitter. The co-axialilluminator achieves efficiencies substantially in excess of thoseachieved with a beamsplitter. In one aspect, an optical stop is used toreflect illumination upon a target. The optical stop includes anaperture allowing light reflected from the target to pass to a detector.The aperture of the stop has an asymmetrical feature that facilitatesoperation on specular targets.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 shows a pick and place machine.

[0010]FIG. 2 is a diagrammatic view of a placement head of a pick andplace machine.

[0011]FIGS. 3A and 3B are diagrammatic views of an imaging systemincluding a co-axial illuminator in accordance with an embodiment of thepresent invention.

[0012]FIG. 4A is a top plan view of a stop in accordance with anembodiment of the present invention.

[0013]FIG. 4B is an enlarged view of an aperture within the stop shownin FIG. 4A

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

[0014] An electronics assembly machine, specifically, a pick and placemachine, is shown generally in FIG. 1. Machine 201 includes conveyorsystem 202 for transporting a printed circuit board 203 into a workingarea. Pick and place machine 201 includes an x and y motor driveassembly 204 for moving placement head 206 independently in the x and ydirections. Attached to head 206 are multiple vacuum nozzles 208, and210 for releasably coupling to one or more components. Head 206 picksone or more components from trays 212 and while head 206 is transportingthe components to printed circuit board 203, sensor 200 senses the x,yand theta orientation of the components. Since sensor 200 is positionedon head 206, it is considered an on-head sensor and senses the x, y andtheta orientation of the components while head 206 transports thecomponents to board 203. FIG. 1 also illustrates camera 209 located onhead 206. Generally, camera 209 is used to image reference marks or“fiducials” or board artwork located on the circuit board 203 in orderto provide location information to machine 201.

[0015]FIG. 2 is a perspective view of placement head 206. Asillustrated, placement head 206 includes a pair of pick and place units214 each of which includes a nozzle 216 that is adapted to releasablycouple a component to be placed such as components 218. Pick and placeunits 214 are adapted to displace components 218 along the Z axis toplace components 218 upon a circuit board 203 (shown in FIG. 1). Whilecomponents 218 are releasably held by nozzles 216, component alignsensor 200 is translated with respect to components 218 while imagingcomponents 218 such that the relative orientations of components 218with respect to circuit board 203 can be computed. FIG. 2 alsoillustrates board align (BA) camera 209 disposed proximate an edge ofplacement head 206 and facing downward. BA camera 209 acquires an imageof a reference position marker (fiducial) on circuit board 203 in orderto allow calculation of the placement head 206 position with respect tocircuit board 203.

[0016]FIG. 3A is a diagrammatic view of board align camera 209 inaccordance with an embodiment of the present invention. Camera 209includes detector 230 which can be any suitable detector including aCharge Coupled Device (CCD) array. Detector 230 is coupled to imageprocessing electronics (not shown) which receive the image detected bydetector 230. Optional filter 232 can be disposed in front of detector230 to filter the image arriving at detector 230. A first pair of optics234A and 234B are positioned in front of filter 232 and are selected tofocus incoming rays upon detector 230. Optics 234A and 234B can beconsidered a first lens cell. Although the first lens cell isillustrated as a pair of optics, the first lens cell can include anysuitable number of lenses, including a single lens. Co-axial illuminator236 is positioned in front of the first lens cell and allows the imageto pass therethrough, while simultaneously injecting illuminationsubstantially along the optical axis of camera 209. Illuminator 236 willbe described in greater detail with respect to FIG. 3B. Optics 238A and238B (also referred to as a second lens cell) are positioned in front ofilluminator 236 and cooperate to receive diverging ray bundles from atarget of interest and bend the rays such that they remain parallel toone another. The second lens cell can also include any suitable numberof lenses including a single lens. The rays remain parallel to oneanother until they pass through the first lens cell where they arecaused to converge upon detector 230. Those skilled in the art willrecognize that the first and second lens cells form a doubly telecentricoptical system that surrounds illuminator 236. FIG. 3A also illustratesoptional darkfield illuminator 240. Darkfield illuminator 240 providesillumination to the target of interest at a relatively high angle ofincidence. While only darkfield illuminator 240 is shown, those skilledin the art will recognize that many other types of illumination could beused as desired.

[0017]FIG. 3B is a diagrammatic view of camera 209 illustratingoperation of the optics. FIG. 3B also shows illuminator 236 in greaterdetail. FIG. 3B illustrates the first lens cell (234A, 234B) receivingrays that are parallel to one another, and bending them slightly suchthat they converge upon detector 230 after passing through filter 232. Apair of baffles 242, 244 are positioned between the first lens cell andco-axial illuminator 236. A stop 246 is included within illuminator 236.Stop 246 includes an aperture 248 through which the rays pass. Stop 246will be shown in greater detail in FIGS. 4A and 4B. One or more lightsources 250 are disposed within enclosure 253. Sources 250 can take theform of Light Emitting Diodes (LED's), incandescent lights, lasers,and/or any other suitable type of light. Source(s) 250 are arranged todirect light toward stop 246. Stop 246 includes a surface 252 thatreflects at least some light substantially along the optical axis ofcamera 209. In some embodiments, surface 252 can be stainless steel andpreferably is diffuse. Surface 252 can also be provided with a diffusereflective coating.

[0018] Illuminator 236 preferably includes an enclosure 253 having areflective interior such that additional rays emanating from source(s)250 are aligned with the optical axis thereby increasing the efficiencyof illuminator 236. In a preferred embodiment, enclosure 253 iscylindrical.

[0019] Optics 238A and 238B form a second lens cell that receivesdiverging rays 254 and bends rays 254 such that they are substantiallyparallel with respect to one another. Thus, the first and second lenscells form an optical system that surrounds co-axial illuminator 236 andis telecentric in object space. However, embodiments of the presentinvention can be practiced advantageously in a non-telecentric opticalsystem as well.

[0020]FIG. 4A is a top plan view of stop 246 in accordance with anembodiment of the present invention. Stop 246 is preferably positionedabove four Light Emitting Diodes 250 (illustrated in dashed lines),however any suitable number of sources may be used. Stop 246 includesaperture 248, which is preferably disposed at the center of stop 246.Surface 252 of stop 246 is a diffuse reflective surface. Thus, lightfrom sources 250 projected normal to surface 252 is reflected offsurface 252 to create the co-axial illuminator. It is feasible to departfrom a normal illumination angle, provided that the light returning byspecular reflection from horizontal target surface 252 is unobstructed.(A horizontal mirror would be an example of a specular horizontal targetsurface.) Suppose that a ray originates in the plane of stop 246non-coincident with the optical axis, and it experiences a specularreflection from horizontal surface 252. By accepted laws of reflection,the ray will return to the plane of stop 246 at a transverse location onstop 246 relative to the optical axis that is directly opposite thepoint of origin. If stop 246 has an aperture that is symmetric about theoptical axis, every ray that originates on the reflective surface ofstop 246 (that is, outside aperture 248) will, on its return, intersectanother opposite point outside aperture 248 and be blocked. Such asystem would therefore undesirably obstruct all specular reflectionsfrom horizontal target surface 252. To overcome this problem, anasymmetry must be introduced into aperture 248. In order to facilitatethis arrangement, tangs 254 are disposed such that substantially noobstructions are present on the opposite side of aperture 248 for anygiven tang. In a preferred embodiment, three tangs 254 are positioned asillustrated in FIG. 4B, though other asymmetrical apertures could beused advantageously. Using embodiments of the present invention allowsuse of a co-axial illuminator with specular targets without requiring abeamsplitter. It will be appreciated by those skilled in the art thatany beamsplitter would attenuate outgoing light, whereas embodiments ofthe present invention avoid such attenuation. The return beam isattenuated by the tangs, but can be arranged to be no worse than theattenuation imposed by a beamsplitter. The efficiency of the system isthereby increased over that achievable using a beamsplitter.

[0021] Although the present invention has been described with referenceto preferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. An illumination and imaging system for acquiringan image of an illuminated target of interest, system comprising: adetector for receiving light and providing an electrical representationof an image; an optical system optically coupled to the detector; a stopdisposed within the optical system and having an aperture for passing aportion of the light therethrough, the stop having an asymmetricalfeature about an optical axis of the stop; and a light source mountedbetween the target and the stop, the light source shining lightsubstantially normally onto a surface of the stop, wherein lightreflected from the surface of the stop onto the target forms lightreceived at the detector.
 2. The system of claim 1, wherein the stop isformed of stainless steel.
 3. The system of claim 1, wherein the stopincludes a diffuse surface.
 4. The system of claim 1, wherein the stopis coated with diffuse reflective material.
 5. The system of claim 1,and further comprising a plurality of additional light sources mountedsubstantially coplanar with the light source.
 6. The system of claim 1,wherein the light source includes a Light Emitting Diode.
 7. The systemof claim 1, and further comprising a plurality of additional lightsource disposed to shine light substantially normally with respect tothe surface of the stop.
 8. The system of claim 1, wherein thetelecentric optical system includes a first lens cell positioned betweenthe detector and the stop.
 9. The system of claim 8, and furthercomprising a baffle positioned between the first lens cell and the stop.10. The system of claim 9, and further comprising at least oneadditional baffle positioned between the baffle and the stop.
 11. Thesystem of claim 1, wherein the stop and the light source are enclosedwithin an enclosure having a reflective interior.
 12. The system ofclaim 11, wherein the enclosure is cylindrically shaped.
 13. The systemof claim 1, wherein the detector is a CCD array.
 14. The system of claim1, wherein the light source is positioned within the telecentric opticalsystem.
 15. The system of claim 1, and further comprising a plurality ofadditional tangs extending into the aperture and arranged asymmetricallyabout the aperture.
 16. The system of claim 1, wherein the opticalsystem is telecentric in object space.
 17. The system of claim 1, andfurther comprising at least one tang extending into the aperture andarranged asymmetrically about the aperture.
 18. An illuminatorcomprising: a enclosure having an optical axis passing therethrough; astop disposed within the enclosure, the stop having a reflective surfaceand an aperture, wherein at least one tang extends into the aperture;and at least one light source disposed within the enclosure and adaptedto direct light toward the reflective surface substantially normally tothe reflective surface.
 19. An optical stop for use with an illuminator,the stop having an aperture alignable with an optical axis of theilluminator, the aperture having a feature therein arrangedasymmetrically within the aperture.